Copper metallizations

ABSTRACT

Copper-based metallizations, useful in the electronics art, comprising copper powder and certain alkali metal/lead borosilicate glass powders. The copper and glass may be dispersed in an inert liquid vehicle, printed on a substrate, and fired in a nitrogen atmosphere to produce conductor patterns. Also the resultant fired conductor patterns.

BACKGROUND OF THE INVENTION

This invention relates to electronic compositions, and moreparticularly, to metallizations useful in producing conductors ondielectric substrates.

Metallizations which are fired onto ceramic dielectric substrates toproduce conductor patterns usually comprise finely divided noble metalsand an inorganic binder, and usually are applied to the substrate as adispersion of the inorganic powders in an inert liquid vehicle. Uponfiring (usually in the range 700°-1000° C.), sintering occurs and themetallic component provides the functional (conductive) utility, whilethe binder (e.g., glass, Bi₂ O₃, etc.) bonds metal particles to thesubstrate and to one another.

There is a need for conductor compositions in which noble metals arereplaced by base metals such as copper, yet having good performancecharacteristics. Copper compositions are usually fired in an inert,nonoxidizing atmosphere (such as nitrogen) to prevent copper fromreacting with oxygen in the air at elevated temperature. Some previousattempts to make copper compositions have used typical glass binderssuch as those having high bismuth oxide, cadmium oxide, or lead oxidecontents. However, it has not been generally recognized that in theabsence of air during firing at 700°-1000° C. copper metal can beoxidized to cuprous oxide, or further to cupric oxide, by reduction ofsome of the constituents of the glass binder. Bismuth oxide and cadmiumoxide have been found to be particularly susceptible to reduction undersuch conditions, lead oxide being less susceptible. Accompanying theoxidation of copper metal to an oxide of copper is the reduction ofbismuth oxide, cadmium oxide or lead oxide to the corresponding metal.

When copper metal is oxidized, the oxide is not solderable and leads tospots in the resultant fired metallization which are not soldered bytypical solders such as Sn/Pb. Although not intended to be limiting, itseems that the reaction of copper with bismuth oxide is generallypreceded by separation of the glass binder into two phases. One is abismuth oxide rich phase which is absorbed by the copper metal surface.The remaining phase, often a lead borosilicate glass, does not wet theabsorbed bismuth oxide phase and adhesion of the metallization isreduced. Tensile strength tests often show the specimens failing in themetallization itself. Where the absorbed layer of bismuth oxide phasehas been reduced to bismuth metal during firing, the metallizations tendto fail by loss of adhesion to the substrate.

The above behavior is most inconvenient since Bi₂ O₃, CdO and PbO arecommon constituents in producing glasses for metallizing compositions,in that they produce glasses of low viscosity and softening point, whichwet the typical alumina dielectric substrates at the relatively lowtemperatures typically used in firing film metallization on substrates.They further provide useful levels of solderability and adhesion to thesubstrate.

SUMMARY OF THE INVENTION

I have discovered a new metallization for producing conductor patternson a dielectric substrate, which although based upon copper, can befired in a nonoxidizing atmosphere such as nitrogen to produce conductorpatterns which exhibit good, reproducible conductivity, adhesion andsolderability. The compositions of this invention comprise 85-97% finelydivided copper powder (preferably 90-97%) and 3-15% finely divided glasspowder (preferably 3-10%), wherein the glass powder is that of Table 1.

                  TABLE 1                                                         ______________________________________                                        GLASSES OF THIS INVENTION                                                     Wt. %                                                                                 Operative  Preferred    Optimum                                       Component                                                                             Range      Range        Range                                         ______________________________________                                        PbO     40-70.sup.(1)                                                                            45-55        45-55                                         PbF.sub.2                                                                              0-20.sup.(1)                                                                             8-13         8-13                                         SiO.sub.2                                                                              7-27       7-20         7-20                                         Al.sub.2 O.sub.3                                                                       0-5        0-1          0-1                                          B.sub.2 O.sub.3                                                                       10-20      12-20        13-18                                         R.sub.2 O (R is                                                                Na,K)  0.25-4.sup.(2)                                                                            1-3 Na.sub.2 O.sup.(4)                                                                     1-3 Na.sub.2 O.sup.(4)                                          0.25-1.5 K.sub.2 O.sup.(4)                                                                 0.25-1.5 K.sub.2 O.sup.(4)                    CeO.sub.2                                                                              0-5.sup.(3)                                                                              1-3%         1-3                                          TiO.sub.2                                                                              0-6.sup.(3)                                                                              0-5%         2-5                                          ______________________________________                                         .sup.(1) Provided that the total weight of PbO plus PbF.sub.2 in the glas     is in the range 50-70%.                                                       .sup.(2) Provided that the wt. of K.sub.2 O is 0-40% of total wt. of          K.sub.2 O plus Na.sub.2 O.                                                    .sup.(3) Provided that where total wt. of CeO.sub.2 plus TiO.sub.2  is        0-1%, there is at least 5% PbF.sub.2 ; and where wt. of PbF.sub.2 is 0-5%     there is at least 1% CeO.sub.2 and/or TiO.sub.2.                              .sup.(4) Provided that total wt. of Na.sub.2 O and/or K.sub.2 O is 0.5-3%                                                                              

The glass may additionally comprise up to 10% MgO, CaO, SrO, BaO, ZrO₂,MnO₂, Fe₂ O₃, CoO, ZnO, and/or As₂ O₃ and/or up to 5% CdO, SnO, Sb₂ O₃,and/or WO₃. Also of this invention are dispersions of these powdercompositions in an inert liquid printing vehicle, as well as firedconductor patterns made with these powder compositions.

DETAILED DESCRIPTION

The essential component in the copper compositions of this invention isa glass of Table 1. These glasses provide excellent fired conductorproperties despite being fired in nonoxidizing atmospheres. The problemsinherent in the use of glasses rich in Bi₂ O₃ and CdO are avoided, suchas limited solder acceptance and failures in adhesion.

PbO is an essential component of these glasses to provide low viscosityand low softening point. At least 40% PbO should be present based on thetotal weight of the glass. In excess of 70% PbO tends to be reduced bydissolved copper under some firing conditions. PbF₂ may optionally bepresent in amounts up to 20%, PbF₂ being a powerful flux in combinationwith PbO and being less reducible by copper metal than is PbO. The totalweight of PbO and PbF₂ in the glass is 50-70%. As PbO is increased, thebeneficial effect of PbF₂ is reduced. The beneficial effect of PbF₂ isdue to its higher anion to cation ratio than is the case in PbO.Preferred glasses comprise 45-55% PbO and 8-13% PbF₂, for minimizedtendency to reduce, yet provide a low softening point.

SiO₂ is 7-27% of the glass, preferably 7-20%. It serves to provideadequate viscosity to reduce spreading of the metallizing binder, andinhibits crystallization and consequent loss of strength.

A small amount of Al₂ O₃ (0-5%, preferably 0-1%) may be present tobalance the effect of the alkali oxides Na₂ O and K₂ O. If Al₂ O₃ isomitted, the chemical durability of the glass versus water becomeslesser and solder leach resistance may decrease.

B₂ O₃ strengthens glass formation although it also tends to reduceviscosity; 10-20% B₂ O₃ is present, preferably 12-20%, more preferably13-18%. The optimum concentration of B₂ O₃ is strongly dependent only onthat of SiO₂, the other "glass-former". Where there is high SiO₂, it ispossible to enhance solderability and adhesion by introducing high Bi₂O₃.

The alkali oxides of Na₂ O and/or K₂ O, although present as only 0.25-4%of the glass, are important constituents, since they introduce singlybonded oxygen atoms, lowering the softening point of the glass andenhancing wetting of the copper and the substrate. They also enhance thesolderability in the glass of some of the more refractory glassconstituents such as Al₂ O₃ and TiO₂. A mixture of both and K₂ O ispreferred due to enhancement of solderability. Optimum mixtures have aNa₂ O/K₂ O weight ratio of about 2/1.

CeO₂ and TiO₂ may be present as 0-5% and 0-6%, respectively, preferably1-3% CeO₂ and 0-5% TiO₂, more preferably 1-3% CeO₂ and 2-5% TiO₂. Smallamounts of TiO₂ enhance acid resistance, although more than 5% increasesrefractoriness. To minimize a tendency toward phase separation andreduction of Ti⁺⁴ to Ti⁺³, CeO₂ is preferably introduced to maintainTiO₂ in the oxidized state during glass melting, as well as to lower thesoftening point.

Other conventional glass constituents may be present in moderate amountswith the exception of those that are easily reduced by such copper atomsas dissolve in the glass. Thus, CdO, SnO, Sb₂ O₃ and WO₃ should totalless than 5% of the glass. Other common glass constituents such as MgO,CaO, SrO, BaO, ZrO₂, MnO, Fe₂ O₃, CoO, ZnO, and/or As₂ O₃ may be presentin fairly large amounts (up to 10% total) without unacceptabledegradation.

The glass of this invention is prepared by conventional glass-makingtechniques, by mixing the desired components (or precursors thereof,e.g., H₃ BO₃ for B₂ O₃) in the desired proportions and heating themixture to form a melt. As is well known in the art, heating isconducted to a peak temperature and for a time such that the meltbecomes entirely liquid, yet gaseous evolution has ceased. In this workthe peak temperature is in the range 1100°-1500° C., usually 1200°-1400°C. The melt is then fritted (particles are made) by cooling the melt,typically by pouring onto a cold belt or into cold running water.Particle size reduction can then be accomplished by milling as desired.

Neither the copper powder specifications nor the method of itspreparation is critical for the purposes of this invention, so long asthe size of the copper powder permits screen printing (0.5-10 m² /g.)and is substantially free of oxide.

The compositions of the present invention comprise finely dividedinorganic powders (copper and glass) dispersed in inert vehicles. Thepowders are sufficiently finely divided to be used in conventionalscreen or stencil printing operations, and to facilitate sintering.Generally, the metallizations are such that at least 90% of theparticles are no greater than 5 microns, and the surface area is 0.5-10m² /g. In optimum metallizations substantially all the particles areless than 10 micron in size.

Other commonly used inorganic powders may be added, so long as theproperty improvements of the present invention are not obviated thereby.

The metallizing compositions are prepared from the solids and vehiclesby mechanical mixing. The metallizing compositions of the presentinvention are printed as a film onto ceramic dielectric substrates inthe conventional manner. Generally, screen stenciling techniques arepreferably employed.

Any inert liquid may be used as the vehicle. Water or any one of variousorganic liquids, with or without thickening and/or stabilizing agentsand/or other common additives, may be used as the vehicle. Exemplary ofthe organic liquids which can be used are the aliphatic alcohols; estersof such alcohols, for example, the acetates and propionates; terpenessuch as pine oil, terpineol and the like; solutions of resins such asthe polymethacrylates of lower alcohols, or solutions of ethylcellulose, in solvents such as pine oil and the monobutyl ether ofethylene glycol monoacetate. The vehicle may contain or be composed ofvolatile liquids to promote fast setting after application to thesubstrate.

The ratio of inert liquid vehicle to solids in the metallizingcompositions of this invention may vary considerably and depends uponthe manner in which the dispersion of metallizing composition in vehicleis to be applied to the kind of vehicle used. Generally, from 0.5 to 20parts by weight of solids per part by weight of vehicle may be used toproduce a dispersion of the desired consistency. Preferred dispersionscontain 10-25 vehicle and 90-75% solids.

In the inorganic solids, there is normally 85-97% copper and 3-15%glass. Lower copper yields fired conductors only difficultly wet bysolder and hence incompletely soldered. Furthermore, conductivity is toolow. Higher copper causes excessive leaching of the copper by solder sothat pattern edge definition and adhesion are reduced. Lower glass givesinadequate adhesion and high glass increases viscosity too much.Preferably, there is 90-97% copper and 3-10% glass.

The metallizing compositions of the present invention are printed ontoceramic substrates, after which the printed substrate is fired to mature(sinter) the metallizing compositions of the present invention, therebyforming continuous conductors on the dielectrics.

The dielectric substrate used in the present invention to makemultilayer capacitors may be any dielectric compatible with theelectrode composition and firing temperature selected, according toprinciples well established in the art. Such dielectrics include bariumtitanate, barium zirconate, lead zirconate, strontium titanate, calciumtitanate, calcium zirconate, lead zirconate, lead zirconate titanate,alumina, etc.

As indicated above, the metallizing compositions of the presentinvention are printed onto ceramic substrates, after which the printedsubstrate is fired in an inert atmosphere (such as nitrogen) to maturethe metallizing compositions of the present invention, thereby formingcontinuous conductors. The printed substrate is fired at a temperaturebelow the melting point of copper (to prevent loss of patterndefinition), at a temperature high enough to mature (sinter) thepattern. These compositions are usually fired at a peak temperature inthe range 700°-1000° C., for 5-15 min. at peak, and preferably are firedat about 900° C. for about 2 hours, 10-15 min. at peak.

EXAMPLES

The following examples and comparative showings are presented toillustrate the advantages of the present invention. In the examples andelsewhere in the specification and claims, all parts, percentages,proportions, etc., are by weight, unless otherwise stated.

Tables 2 and 3 list the compositions of melted glasses examined in thiswork, Table 2 listing various glasses of this invention and Table 3glasses outside this invention. Softening points are listed. In the caseof the glasses of Table 3, observations as to unacceptability of eachglass are noted.

                                      TABLE 2                                     __________________________________________________________________________    MELTED GLASS COMPOSITIONS OF THIS INVENTION                                          Glass No. (Wt. %)                                                      Component                                                                            1  2  3  4  5  6  7  8  9  10                                          __________________________________________________________________________    PbO    47.6                                                                             51.9                                                                             63.0                                                                             63.0                                                                             47.7                                                                             47.1                                                                             46.7                                                                             46.7                                                                             52.7                                                                             54.7                                        PbF.sub.2                                                                            11.1                                                                             10.0                                                                             -- -- 11.1                                                                             11.1                                                                             11.1                                                                             11.1                                                                             11.1                                                                             11.1                                        SiO.sub.2                                                                            21.6                                                                             24.4                                                                             17.4                                                                             15.4                                                                             19.3                                                                             17.1                                                                             17.2                                                                             17.2                                                                             11.2                                                                             9.2                                         Al.sub.2 O.sub.3                                                                     0.7                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.7                                                                              0.7                                                                              0.7                                                                              0.7                                                                              0.7                                                                              0.7                                         B.sub.2 O.sub.3                                                                      16.7                                                                             12.0                                                                             15.0                                                                             15.0                                                                             16.7                                                                             14.4                                                                             14.4                                                                             14.4                                                                             14.4                                                                             14.4                                        Na.sub.2 O                                                                           1.7                                                                              1.1                                                                              2.0                                                                              2.0                                                                              1.7                                                                              1.7                                                                              2.2                                                                              2.2                                                                              2.2                                                                              2.2                                         K.sub.2 O                                                                            0.6                                                                              -- -- -- 0.6                                                                              0.6                                                                              1.1                                                                              1.1                                                                              1.1                                                                              1.1                                         CeO.sub.2                                                                            -- -- 2.0                                                                              -- 2.2                                                                              2.2                                                                              2.2                                                                              2.2                                                                              2.2                                                                              2.2                                         TiO.sub.2                                                                            -- -- -- 4.0                                                                              -- 4.4                                                                              4.4                                                                              4.4                                                                              4.4                                                                              4.4                                         __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    MELTED GLASS COMPOSITIONS NOT OF THIS INVENTION (COMPARATIVE) - wt.           __________________________________________________________________________    Component                                                                           A   B  C   D  E  F  G  H  I  J  K  L  M  N                              __________________________________________________________________________    PbO   73.3                                                                              78.0                                                                             63.0                                                                              53.0                                                                             43.0                                                                             43.0                                                                             43.0                                                                             42.0                                                                             60.4                                                                             56.4                                                                             51.4                                                                             68 62 28                             PbF.sub.2                                                                           --  -- --  -- 10.0                                                                             10.0                                                                             10.0                                                                             10.0                                                                             -- -- -- -- -- --                             SiO.sub.2                                                                           15.4                                                                              9.4                                                                              19.4                                                                              19.4                                                                             19.4                                                                             17.4                                                                             15.4                                                                             15.4                                                                             24.4                                                                             24.4                                                                             24.4                                                                             19.4                                                                             19.4                                                                             19.4                           Al.sub.2 O.sub.3                                                                    0.6 0.6                                                                              0.6 0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                            B.sub.2 O.sub.3                                                                     9.6 10.0                                                                             15.0                                                                              15.0                                                                             15.0                                                                             15.0                                                                             13.0                                                                             13.0                                                                             12.0                                                                             12.0                                                                             12.0                                                                             10.0                                                                             15.0                                                                             50.0                           Na.sub.2 O                                                                          1.1 2.0                                                                              2.0 1.5                                                                              1.5                                                                              1.5                                                                              1.5                                                                              2.0                                                                              1.1                                                                              1.1                                                                              1.1                                                                              2.0                                                                              3.0                                                                              1.5                            K.sub.2 O                                                                           --  -- --  0.5                                                                              0.5                                                                              0.5                                                                              0.5                                                                              1.0                                                                              -- -- -- -- -- 0.5                            CeO.sub.2                                                                           --  -- --  -- -- 2.0                                                                              2.0                                                                              2.0                                                                              -- -- -- -- -- --                             TiO.sub.2                                                                           --  -- --  -- -- -- 4.0                                                                              4.0                                                                              -- -- -- -- -- --                             Bi.sub.2 O.sub.3                                                                    --  -- --  1.0                                                                              10.0                                                                             10.0                                                                             10.0                                                                             10.0                                                                             -- -- -- -- -- --                             CdO   --  -- --  -- -- -- -- -- 1.0                                                                              5.0                                                                              10.0                                                                             -- -- --                                   .BHorizBrace.                                                                        too .BHorizBrace.  .BHorizBrace.                                                                          .BHorizBrace.                        problem                                                                             Pb reduction                                                                         hard*                                                                             Bi reduction   Cd reduction                                                                           too hard                             __________________________________________________________________________     *Means not fluid enough when softened.                                   

The glasses were prepared from batches of the respective compounds orprecursors thereof (litharge for PbO, flint for SiO₂, Al₂ O₃.H₂ O forAl₂ O₃, Na₂ CO₃ for Na₂ O, B₂ O₃, 85% K₂ CO₃ for K₂ O, PbF₂, CeO₂ andTiO₂) and were best melted for 20-30 min. in platinum vessels to1150-1200° C. Surface area of glasses used in these examples was 2-5m.²/g.

The copper powder had a surface area of about 1 m² /g. and wasessentially free of oxygen.

Table 4 indicates the glass used, and the glass/copper vehicle ratioused, in producing fired conductors (on dense alumina substrates)according to this invention. The composition was printed through a 325-mesh screen onto a dense alumina substrate in the following pattern: Aserpentine pattern was printed, as was a pattern of nine 200-mil squareopenings in a 3×3 matrix. The printed substrate was then dried at 110°C. for 10 min. and fired in nitrogen in a belt furnace to a peaktemperature of 900° C., using a 2-hour program with 10 min. at peak. Thefired conductor patterns were about 1 mil thick.

Conductance was determined on the serpentine pattern using a FlukeInstrument Co. Model 8800A D.C. ohmmeter.

Solderability was determined by dipping the pattern into a 60Sn/40Pbsolder bath at 215° C.

Adhesion was found by placing a 20-gauge pretinned copper wire acrossthree of the fired 200-mil square pads and then soldering, followed bypulling the soldered leads at 90° to the surface of the soldered padwith a Chatillon spring tester.

                  TABLE 4                                                         ______________________________________                                        COPPER METALLIZING COMPOSITIONS                                                      Number of Table 2 Glass Used                                           Composition                                                                   (wt. %): 9      1      7    6    2    3    4    5                             ______________________________________                                        Copper   80     77.3   77.3 77.9 80   77.3 80.4 76.7                          Glass    6.2    6.0    6.0  5.4  6.2  6.0  2.9  6.7                           Ethyl                                                                         cellulose/                                                                    terpineol, 1/9                                                                         13.8   16.7   16.7 16.7 13.8 16.7 16.7 16.7                          Fired Conductor Properties:                                                   Conductance                                                                            1.7    2.5    2.3  3.4  3.5  4.0  3.2  4.3                           (milliohms/                                                                    sq.)                                                                         Solderability                                                                          exc.   fair   good fair good fair fair fair                          Adhesion 5.5    5.3    5.1  3.4  4.5  3.0  4.0  4.6                           (lb./in..sup.2)                                                               ______________________________________                                    

I claim:
 1. In compositions of finely divided metal powder and glassbinder powder useful for forming conductor patterns on a dielectricsubstrate, wherein the improvement consists essentially of, as the metalpowder, 85- 97 weight % copper powder and, as the glass binder powder,3-15 weight % of a glass powder comprising40-70% PbO 0-20% pbF₂ 7-27%siO₂ 0-5% al₂ O₃ 10-20% b₂ o₃ 0.25-4% m₂ owherein M is Na, K andmixtures thereof, provided that the weight of K₂ O is 0-40% of the totalweight of K₂ O and Na₂ O, 0-5% ceO₂ 0-6% tiO₂,provided that (1) wherethe total weight of CeO₂ and TiO₂ is 0-1%, there is at least 5% PbF₂present; (2) where the weight of PbF₂ is 0-5%, there is at least 1%CeO₂, TiO₂ or mixtures thereof, and (3) the total weight of PbO and PbF₂is 50-70%.
 2. Compositions according to claim 1 wherein the glasscomprises45-55% PbO 8-13% pbF₂ 7-20% siO₂ 0-1% al₂ O₃ 12-20% b₂ o₃ 1-3%na₂ O 0.25-1.5 % k₂ o,provided that the total weight of Na₂ O and K₂ Ois 0.5-3% 1-3% CeO₂ 0-5 % tiO₂.
 3. Compositions according to claim 3wherein the glass comprises45-55% PbO 8-13% pbF₂ 7-20% siO₂ 0-1% al₂ O₃13-18% b₂ o₃ 1-3% na₂ O 0.25-1.5% k₂ o,provided that the total weight ofNa₂ O and K₂ O is 0.5-3% 1-3% CeO₂ 2-5% tiO₂.
 4. Compositions accordingto claim 1 wherein the glass additionally comprises up to 10% MgO, CaO,SrO, BaO, ZrO₂, MnO, Fe₂ O₃, CoO, ZnO, and/or AS₂ O₃.
 5. Compositionsaccording to claim 2 wherein the glass additionally comprises up to 10%MgO, CaO, SrO, BaO, ZrO₂, MnO, Fe₂ O₃, CoO, ZnO, and/or As₂ O₃. 6.Compositions according to claim 3 wherein the glass additionallycomprises up to 10% MgO, CaO, SrO, BaO, ZrO₂, MnO, Fe₂ O₃, CoO, ZnO,and/or As₂ O₃.
 7. Compositions according to claim 1 wherein the glassadditionally comprises up to 5% CdO, SnO, Sb₂ O₃, and/or WO₃. 8.Compositions according to claim 2 wherein the glass additionallycomprises up to 5% CdO, SnO, Sb₂ O₃, and/or WO₃.
 9. Compositionsaccording to claim 3 wherein the glass additionally comprises up to 5%CdO, SnO, Sb₂ O₃, and/or WO₃.
 10. Compositions according to claim 1 of90-97% copper powder and 3-10% glass powder.
 11. Compositions accordingto claim 1 dispersed in an inert liquid vehicle.
 12. Compositionsaccording to claim 2 dispersed in an inert liquid vehicle. 13.Compositions according to claim 3 dispersed in an inert liquid vehicle.14. A dielectric substrate bearing a conductor pattern of thecomposition of claim
 1. 15. A dielectric substrate bearing a conductorpattern of the composition of claim
 2. 16. A dielectric substratebearing a conductor pattern of the composition of claim 3.